The present invention relates to a tungsten carbide/zeolite catalyst which has been found to be effective as a hydrogenation catalyst.
More specifically, the present invention relates to a tungsten carbide/zeolite catalyst which has been found to be effective as a hydrogenation catalyst in the conversion of unsaturated sulfur containing compounds to saturated sulfur containing compounds, such as in the conversion of thiophene to tetrahydrothiophene.
There exist known catalysts which are capable of converting sulfur containing unsaturated compounds including thiophene in the presence of H.sub.2 to yield principally H.sub.2 S and hydrocarbons as the main products. Generally, however, little or no tetrahydrothiophene is formed using such accepted catalysts in the conversion of thiophene. For example, using such known catalysts, amounts in the order of 1-2% of tetrahydrothiophene may be produced.
Catalysts which have been heretofore available are primarily designed for the hydrodesulfurization of feed stocks in the petrochemical industry. The main hydrocarbons formed are olefinic in nature, indicating that the reaction mechanism involves mainly breaking the C--S bond into H.sub.2 S and olefins, with lesser hydrogenation activity leading to tetrahydrothiophene and parafins. See for example (J. of Cat. 126,643 (1990).
Other known catalysts that could be employed to reduce sulfur containing components without breaking the C--S bond are palladium on carbon or palladium on barium sulfate, (Mozingo, et al. J. Amer. Chem. Soc., 67, 2092 (1945). However, this reduction reaction is carried out in the presence of mineral acid and can only be used in single batches due to sulfur poisoning of the palladium.
Catalysts such as Mo.sub.2 N have also been reported as hydrodesulfurization catalysts, (Markel & Van Zee, J. of Cat., 126, 643 (1990). Using such a catalyst the thiophene cleaves at the C--S bond. The main hydrocarbon products from this reaction are unsaturated olefinic isomers.
Tetrahydrothiophene is a commercial material employed primarily for odorizing household cooking gas in order to alert persons for safety reasons in case of gas leakage. Derivatives of tetrahydrothiophene are also used in the manufacture of pharmaceuticals.
A tungsten/zeolite catalyst has been found which is effective in the hydrogenation of unsaturated sulfur containing compounds and particularly in the selective hydrogenation of thiophene to tetrahydrothiophene, that is to say, without appreciably breaking the C--S bond, according to the following: ##STR1##
Various references exist in the prior art of which the applicants are aware which relate to the hydrogenation of thiophenes. The most significant of these are the following:
Hargreaves et al (J. of Catalysis 56,363-376 (1979), teach that the hydrogenation of thiophene occurs prior to C--S bond cleavage and that both effects are promoted by Co.
Lee et al (Appl. Cat. 19,207-210 (1985) teaches the effects of thiophene hydrodesulfurization over unsupported molydenum carbide.
Dockner (Angew. Chem., Int., Ed. Engl. 27,679-682 (1988), deals with the hydrogenation of olefins and ureas with technical white oil and activated carbon, amongst other reactions pertinent to the conversion of various hydrocarbons available from petroleum and natural gas as raw materials.
Beck, U.S. Pat. No. 5,120,692 which issued on Jun. 9, 1992, is directed to molecular sieves coated with non-oxide ceramics, which are indicated as being potentially useful as hydrogenation catalysts when used in conjunction with a hydrogenating component such as, among others, tungsten.
These references, however, teach nothing with regard to the specific advantages to be achieved via the utilization of a tungsten carbide/zeolite catalyst according to the present invention, especially in the hydrogenation of thiophene.
The shortcomings of the prior art catalysts noted above are overcome by using the catalyst described in the present invention which allows one to effect the hydrogenation of unsaturated sulfur containing compounds and to effect the production of tetrahydrothiophene in substantial quantities directly from thiophene, without producing the large quantities of H.sub.2 S and hydrocarbon by-products which have heretofore been the result of such reactions using available prior art catalysts.
It is therefore the object of this present invention to provide for a novel hydrogenation catalyst.
It is a further object of the present invention to provide a process for the hydrogenation of unsaturated sulfur containing compounds without substantially cleaving the C--S bonds present in such compounds.
It is also an object of the present invention to provide an improved process for the selective production of tetrahydrothiophene directly from thiophene.
Lastly, it is an object of the present invention to provide an improved process for the production of tetrahydrothiophene using the catalyst of the present invention to selectively produce such tetrahydrothiophene with substantial yield, directly from thiophene.
These and other objects of the invention will be reflected in the course of the following more detailed discussion.